FIG. 1 illustrates a wind turbine 1, comprising a tower 2 on which a nacelle 3 is mounted. At least one turbine blade 5 is mounted on a hub 6 to form a rotor 4. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The wind turbine illustrated in FIG. 1 may be a small model intended for domestic or light utility usage, or may be a large model, such as those that are suitable for use in large scale electricity generation on a wind farm. In the latter case, the diameter of the rotor may be as large as 150 metres or more.
The rotor blades of wind turbines are designed to extract power from the wind by virtue of their aerodynamic shape, and subsequent wind induced rotation. For horizontal axis wind turbines, the blades rotate around a rotor hub attached to a nacelle mounted on a wind turbine tower, and the rotation of the rotor turns a drive shaft connected in turn to a generator which produces electricity. For horizontal axis wind turbines to operate efficiently and extract the maximum power from the wind, the wind turbine nacelle, and the axis around which the wind turbine rotor rotates, is angled into the wind to the greatest extent possible such that the rotational axis of the rotor is aligned with the wind direction.
Modern wind turbines control the load on the rotor by pitching the blades in and out of the incoming wind. The blades are pitched to optimize the output or to protect the wind turbine from damaging overloads during high winds. FIG. 2 shows a known pitching arrangement between a hub 3 and a blade (not shown). The pitch bearing 7 is located between the hub and the blade and in modern turbines can have a diameter as large as 2 metres up to 6 metres. A gear 9 is formed on the pitch bearing 7, usually by machining the gear ring into the pitch bearing material, and a torque is applied by a pinion 8 to the gear to turn the pitch bearing. The pinion is turned by one or more actuators, such as a hydraulic cylinder or electric motor, to provide the torque for pitching the blade and maintaining it in a given position. Such a pitching arrangement enables each blade to be turned approximately 90°-100° around their longitudinal axis.
A problem with pitch bearings for wind turbines is that they are not operated like traditional gear systems. The gear goes through long periods of inactivity, in which it is not necessary to change the pitch of the blades. When the blades are pitched, only relatively small rotations are required to control the power and speed of the rotor when the turbine is being used to extract energy from oncoming wind, this being within what will be called the “operational range” for pitching. The result is that the pinion spends most of the time engaged with, and travelling over, a short section of the gear surface. Typically this equates to around 30°-35° of the gear surface, defined by the angle of a segment of the gear ring. This leads to accelerated wear, and when the small section of gear covering adjustments in the operational range becomes worn the entire gear must be replaced. Because the gear is formed directly on the blade bearing the entire bearing, or bearing ring, must be replaced, which is expensive and difficult.
We have appreciated that it would be advantageous to provide a gear for a turbine pitch system that can be easily replaced.